otkuda

1. where from; from where
   Otkuda si ti, iz Beograda? — Where are you from, Belgrade?
   Ne znam otkuda dolazi ta buka. — I don't know where that noise is coming from.
2. (in questions) how come; how
   Otkuda znaš moj broj telefona? — How do you know my phone number?
   Otkuda ti ideja da je on kriv? — What makes you think he's to blame?

In speech the short form otkud is very common.